Here is the code for simple modal popup using Javascript which works in sharepoint as well as ASP.NET
<div id='modal' style='position: absolute; top: 0px; left: 0px; display: none; z-index: 9;
background-color: #383838; filter: alpha(opacity=60);'>
</div>
<div id='chart' style='position: absolute; top: 100px; left: 180px; display: none;
z-index: 10;'>
<asp:Button ID="btnYes" runat ="server" Text ="Yes" OnClick ="btnYes_Click" />
<button onclick="javascript:document.getElementById('modal').style.display='none';document.getElementById('chart').style.display='none';">
CANCEL</button>
<br />
<br />
</div>
<a href='#' onclick="javascript:document.getElementById('modal').style.width = document.body.clientWidth + 'px';document.getElementById('modal').style.height = document.body.clientHeight + 'px';document.getElementById('modal').style.display='block';document.getElementById('chart').style.display='block';">
<asp:Label ID="lblClick" runat ="server" Text ="Click Me"></asp:Label>
</a>
To get this code working properly in ASP.NET remove the following code in .aspx pages
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
